Calculate the runtime of system() command in C using clock() function

Question

I am trying to calculate the time taken by the system command in my C program. I am trying to call a MATLAB routine from C using system() function within C. I want to calculate how much cputime is consumed from the time system() is called, MATLAB runs, exits and returns control back to C. My pseudocode snippet is below:

void main()
{
    clock_t matlab_begin, matlab_end;
    matlab_begin = clock();

    system(/String for invoking MATLAB routine/);
    printf("MATLAB ENDED");

    matlab_end = clock();

    total_time_taken = (float)(matlab_end - matlab_begin)/CLOCKS_PER_SEC;
}

The value of total_time_taken comes to 0.03 sec. Whereas when I calculate the cputime taken to run my MATLAB script from the start of the MATLAB routine till the end within MATLAB using cputime function, I end up getting around 11 sec.

I want to know why are the two runtimes so different? Also the print statement just after the system() command does not get executed until MATLAB finishes its job and exits and gives control back to the system command.. So, ideally both the runtime should come out to be the same, but they do not!

Pls help! Thanks.

Answer 1

I believe that the clock call in your C code is only getting the time for you C program. The system call is launching another program (in this case Matlab). You're not seeing any of Matlab's execution time when you get the result from clock in your C program. That 0.03 sec is just the time it takes to start your main program, call clock, and launch Matlab.

See also The C `clock()` function just returns a zero